1. Field of the Invention
The present invention relates to a micro-electromechanical system (MEMS) microphone structure and a method of fabricating the same, and more particularly, to a method of fabricating a MEMS microphone structure and the MEMS microphone structure formed thereby so as to effectively reduce the thickness of the MEMS microphone structure.
2. Description of the Prior Art
MEMS microphone structures are modern technology, which coordinate electrical circuits and mechanics. The MEMS microphone structure presently can be made from general micro electronic technology, such as photolithography, vapor phase deposition, etching or LIGA, performed on the substrate, such as an insulating layer or other semiconductor. Recently, in order to coordinate the MEMS device and the complementary metal-oxide-semiconductor (CMOS) device into MEMS microphone structure, MEMS microphone structure is fabricated by using the same types of steps that are used to fabricate conventional analog and digital complementary metal oxide semiconductor (CMOS) circuits.
Referring to FIG. 1 through FIG. 3, FIG. 1 through FIG. 3 are schematic diagrams illustrating a method of fabricating a MEMS microphone structure according to the prior art. As shown in FIG. 1, the method of fabricating the MEMS microphone structure 10 with multi-metal layers according to the prior art is as follows. First, a substrate 12 is provided, and the surface of the substrate 12 has a base sacrificial layer 14 and a first metal layer 16. Then, the first metal layer 16 is patterned to form a first micro-machined metal mesh 18. Next, as shown in FIG. 2, a first sacrificial layer 20 is deposited to cover the substrate 12, and the surface of the first sacrificial layer 20 is planarized. Thereafter, a second metal layer 22 is formed on the first sacrificial layer 20, and then, a second metal layer 22 is patterned to form a second micro-machined metal mesh. Next, a second sacrificial layer 26 is deposited to cover the substrate 12, and the surface of the second sacrificial layer 26 is planarized. Then, a third metal layer 28 is formed on the second sacrificial layer 26, and then, the third metal layer 28 is patterned to form a third micro-machined metal mesh. Next, a third sacrificial layer 32 covers the substrate 12. Finally, as shown in FIG. 3, an isotropic dry etching process is utilized to remove the first sacrificial layer 20, the second sacrificial layer 26, the third sacrificial layer 32 and a part of the base sacrificial layer 14 among the first micro-machined metal mesh 18, second micro-machined metal mesh and third micro-machined metal mesh, so that the first micro-machined metal mesh 18, the second micro-machined metal mesh and the third micro-machined metal mesh are suspended above the substrate 12 so as to form a multilayer membrane. Next, a back side etching process is performed to etch through the substrate 12 so as to allow free movement of air molecules and vibrate the multilayer membrane. Therefore, the MEMS microphone structure 10 according to the prior art is formed.
As mentioned above, the micro-machined metal mesh according to the prior art is formed by using an Al process. The first metal layer, the second metal layer and the third metal layer composed of aluminum are respectively formed first, and then, a photoresist layer is coated thereon. Next, the photoresist layer is patterned, and then, the photoresist layer is used as a mask to etch the metal layers so as to form the micro-machined metal mesh. However, the height between the micro-machined metal mesh and the substrate depends on the thickness of the sacrificial layer, so that the thickness of the MEMS microphone structure is the total thickness of the sacrificial layers and the micro-machined metal meshes. The thickness of the MEMS device is limited. Therefore, in order to make the MEMS microphone structure have a smaller size or thickness, to improve the MEMS microphone structure and the method of fabricating the same is important.